I see what you're saying, but surely the mileage the engine has done would count more than the overal age of the engine?

You could say that, but seals will go bad when a car sits without being turned on over time just as they do when the car is driven. An older car makes good use of the impurities and junk found in dino oil to keep aged seals from becoming sprinklers. Try synthetic and theres a good chance you'll start popping $10 every week to top off your oil.